# std::count, std::count_if

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< cpp‎ | algorithm

Defined in header `<algorithm>` |  |
| (1) |
template< class InputIt, class T > typename std::iterator_traits<InputIt>::difference_type count( InputIt first, InputIt last, const T& value ); |  | (constexpr since C++20) (until C++26)
template< class InputIt, class T = typename std::iterator_traits <InputIt>::value_type > constexpr typename std::iterator_traits<InputIt>::difference_type count( InputIt first, InputIt last, const T& value ); |  | (since C++26)
| (2) |
template< class ExecutionPolicy, class ForwardIt, class T > typename std::iterator_traits<ForwardIt>::difference_type count( ExecutionPolicy&& policy, ForwardIt first, ForwardIt last, const T& value ); |  | (since C++17) (until C++26)
template< class ExecutionPolicy, class ForwardIt, class T = typename std::iterator_traits <ForwardIt>::value_type > typename std::iterator_traits<ForwardIt>::difference_type count( ExecutionPolicy&& policy, ForwardIt first, ForwardIt last, const T& value ); |  | (since C++26)
template< class InputIt, class UnaryPred > typename std::iterator_traits<InputIt>::difference_type count_if( InputIt first, InputIt last, UnaryPred p ); | (3) | (constexpr since C++20)
template< class ExecutionPolicy, class ForwardIt, class UnaryPred > typename std::iterator_traits<ForwardIt>::difference_type count_if( ExecutionPolicy&& policy, ForwardIt first, ForwardIt last, UnaryPred p ); | (4) | (since C++17)

Returns the number of elements in the range `[`first`,` last`)` satisfying specific criteria.

1) Counts the elements that are equal to value (using operator==).
3) Counts elements for which predicate p returns true.
2,4) Same as (1,3), but executed according to policy.
These overloads participate in overload resolution only if all following conditions are satisfied:

std::is_execution_policy_v<std::decay_t<ExecutionPolicy>> is true. | (until C++20)
std::is_execution_policy_v<std::remove_cvref_t<ExecutionPolicy>> is true. | (since C++20)

### Parameters

first, last | - | the range of elements to examine
value | - | the value to search for
policy | - | the execution policy to use
p | - | unary predicate which returns ​true for the required elements. The expression p(v) must be convertible to bool for every argument `v` of type (possibly const) `VT`, where `VT` is the value type of `InputIt`, regardless of value category, and must not modify `v`. Thus, a parameter type of VT&is not allowed, nor is VT unless for `VT` a move is equivalent to a copy(since C++11). ​
Type requirements
- `InputIt` must meet the requirements of LegacyInputIterator.
- `ForwardIt` must meet the requirements of LegacyForwardIterator.
- `UnaryPred` must meet the requirements of Predicate.

### Return value

The number of iterators it in the range `[`first`,` last`)` satisfying the following condition:

1,2) *it == value is true.
3,4) p(*it) != false is true.

### Complexity

Given \(\scriptsize N\)N as std::distance(first, last):

1,2) Exactly \(\scriptsize N\)N comparisons with value using operator==.
3,4) Exactly \(\scriptsize N\)N applications of the predicate p.

### Exceptions

The overloads with a template parameter named `ExecutionPolicy` report errors as follows:

- If execution of a function invoked as part of the algorithm throws an exception and `ExecutionPolicy` is one of the standard policies, std::terminate is called. For any other `ExecutionPolicy`, the behavior is implementation-defined.
- If the algorithm fails to allocate memory, std::bad_alloc is thrown.

### Notes

For the number of elements in the range `[`first`,` last`)` without any additional criteria, see std::distance.

Feature-test macro | Value | Std | Feature
--- | --- | --- | ---
`__cpp_lib_algorithm_default_value_type` | `202403` | (C++26) | List-initialization for algorithms (1,2)

### Possible implementation

See also the implementations of `count` in libstdc++ and libc++.

See also the implementations of `count_if` in libstdc++ and libc++.

count
    template<class InputIt, class T = typename std::iterator_traits<InputIt>::value_type>
    typename std::iterator_traits<InputIt>::difference_type
        count(InputIt first, InputIt last, const T& value)
    {
        typename std::iterator_traits<InputIt>::difference_type ret = 0;
        for (; first != last; ++first)
            if (*first == value)
                ++ret;
        return ret;
    }
count_if
    template<class InputIt, class UnaryPred>
    typename std::iterator_traits<InputIt>::difference_type
        count_if(InputIt first, InputIt last, UnaryPred p)
    {
        typename std::iterator_traits<InputIt>::difference_type ret = 0;
        for (; first != last; ++first)
            if (p(*first))
                ++ret;
        return ret;
    }

### Example

Run this code

    #include <algorithm>
    #include <array>
    #include <cassert>
    #include <complex>
    #include <iostream>
    #include <iterator>

    int main()
    {
        constexpr std::array v{1, 2, 3, 4, 4, 3, 7, 8, 9, 10};
        std::cout << "v: ";
        std::copy(v.cbegin(), v.cend(), std::ostream_iterator<int>(std::cout, " "));
        std::cout << '\n';

        // Determine how many integers match a target value.
        for (const int target : {3, 4, 5})
        {
            const int num_items = std::count(v.cbegin(), v.cend(), target);
            std::cout << "number: " << target << ", count: " << num_items << '\n';
        }

        // Use a lambda expression to count elements divisible by 4.
        int count_div4 = std::count_if(v.begin(), v.end(), [](int i) { return i % 4 == 0; });
        std::cout << "numbers divisible by four: " << count_div4 << '\n';

        // A simplified version of `distance` with O(N) complexity:
        auto distance = [](auto first, auto last)
        {
            return std::count_if(first, last, [](auto) { return true; });
        };
        static_assert(distance(v.begin(), v.end()) == 10);

        std::array<std::complex<double>, 3> nums{{{4, 2}, {1, 3}, {4, 2}}};
        #ifdef __cpp_lib_algorithm_default_value_type
            // T gets deduced making list-initialization possible
            auto c = std::count(nums.cbegin(), nums.cend(), {4, 2});
        #else
            auto c = std::count(nums.cbegin(), nums.cend(), std::complex<double>{4, 2});
        #endif
        assert(c == 2);
    }

Output:

    v: 1 2 3 4 4 3 7 8 9 10
    number: 3, count: 2
    number: 4, count: 2
    number: 5, count: 0
    numbers divisible by four: 3

### Defect reports

The following behavior-changing defect reports were applied retroactively to previously published C++ standards.

DR | Applied to | Behavior as published | Correct behavior
--- | --- | --- | ---
LWG 283 | C++98 | `T` was required to be EqualityComparable, but the value type of `InputIt` is not always `T` | removed the requirement

### See also

distance | returns the distance between two iterators (function template)
ranges::countranges::count_if (C++20)(C++20) | returns the number of elements satisfying specific criteria (algorithm function object)

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